mir/vm: SSA pin+PHI + short-circuit; user-defined method calls → functions; entry single-pred PHIs; compare-operand pin; VM BoxCall fallback to InstanceBox methods; docs: update CURRENT_TASK (plan + acceptance)

- Lower And/Or to branch+PHI (RHS not evaluated)
- Always slotify compare operands (dominance safety)
- Insert single-predecessor PHIs at then/else/short-circuit entries
- pin_to_slot now logs (NYASH_PIN_TRACE) and participates in PHI
- Rewrite user-defined instance method calls to Box.method/Arity (builder)
- VM fallback: BoxCall on InstanceBox dispatches to lowered functions with 'me'+args
- Keep plugin/BoxCall path for core boxes (String/Array/Map)
- Add env-gated pre-pin for if/loop (NYASH_MIR_PREPIN)
- CURRENT_TASK: add SSA/userbox plan, debug steps, acceptance criteria

src/backend/mir_interpreter/helpers.rs

@@ -38,6 +38,11 @@ impl MirInterpreter {
         use BinaryOp::*;
         use VMValue::*;
         Ok(match (op, a, b) {
+            // Safety valve: treat Void as 0 for + (dev fallback for scanners)
+            (Add, VMValue::Void, Integer(y)) => Integer(y),
+            (Add, Integer(x), VMValue::Void) => Integer(x),
+            (Add, VMValue::Void, Float(y)) => Float(y),
+            (Add, Float(x), VMValue::Void) => Float(x),
             (Add, Integer(x), Integer(y)) => Integer(x + y),
             (Add, String(s), Integer(y)) => String(format!("{}{}", s, y)),
             (Add, String(s), Float(y)) => String(format!("{}{}", s, y)),